The present invention relates generally to a data storage apparatus employing disklike record media such as magnetic disks notably including those of the "fixed" or hard disk variety. More particularly, my invention pertains to a method for the closed-loop servo positioning of a data transducer with respect to a multiplicity or plurality of addressable annular data tracks formed concentrically on at least one of the major surfaces of the data storage disk in such apparatus.
Three typical methods are known which have been conventionally used for positioning the transducer with respect to the data tracks on the hard magnetic disk. They are: (a) a servo system with a servo transducer and a replicated servo surface on the disk; (b) a servo system employing an encoder for the determination of the positional relationship between the transducer and the data tracks; and (c) the open-loop system with a stepper motor for incrementing the transducer in response to stepping pulses.
The first recited servo positioning system has the disadvantage that only one side of each disk can be used for data storage, the other side being devoted exclusively to the servo control positioning of the transducer. Another drawback is the high cost of the equipment needed for creating the servo control information on the disk. The second mentioned servo system is also objectionable because of the high cost of the encoder, which is very complex in construction. The third, open-loop system is unsatisfactory in the speed of seek operation.
All these drawbacks are absent from another known scheme utilizing addresses prerecorded on individual tracks on the disk. In seeking any of such addressed tracks the transducer reads the prerecorded addresses until it becomes positioned on the destination track. Although capable of accurately positioning the transducer on any desired track, this scheme has the weakness that the track addresses demand a greater number of bits for identification of individual tracks as the track density per disk is increased, as is the current trend. Consequently, with the appearance of such greater track density disks, it has become more and more difficult to move the transducer across the closely spaced tracks at sufficiently high speed as the transducer must read the addresses composed of increasingly greater numbers of bits.
The copending application Ser. No. 358,670 cross-referenced above represents a solution to this problem. It teaches to divide all the data tracks on each major surface of the disk into a plurality of groups and to assign the same set of address code characters to all the tracks of each group. There must therefore be only as many different address code characters as the number of tracks (e.g. sixteen) in each group. Each code character can be constituted of a far less number of code elements (e.g. discretely magnetized and nonmagnetized regions on the disk) than if different code characters were used for all the tracks on the disk, as had been the case with the more conventional track address scheme. No code characters are needed for discrimination of the track groups; only, the code characters for the individual tracks of each group may be formulated with a rule such that the track groups are identifiable from the address code characters that are read during each track seek operation.
I have, however, found a weakness in this prior art addressable transducer positioning system. As the transducer travels across the data tracks for track seeking, the address code characters being read from the successive tracks by the transducer are sampled at regular intervals, and the samples are delivered to the microprocessor included in the positioning system. The microprocessor of the prior art positioning system has required at least one sample code character from each track group being traversed by the transducer, in order to identify the successive track groups. Should the transducer be moved too fast across the tracks, it might traverse one complete track group with no sample code character delivered therefrom to the microprocessor. The microprocessor would then fail to recognize the successive track groups being traversed and, as a result, to position the transducer on the destination track.
For a given sampling interval, the higher the traveling speed of the transducer, the longer is the distance the transducer travels during each sampling interval. Consequently, according to the prior art system, the traveling speed of the transducer has had to be sufficiently low to assure the delivery of at least one sample code character from each track group being traversed. It is thus seen that for higher speed track seeking, the prior art system must be improved so that the transducer can be accurately positioned on the destination track even if no sample code character is fed to the microprocessor from any one or more complete track groups that have been traversed by the transducer during each seek operation.